UPE grades are linear polyethylenes obtained by the low pressure process and having an average molecular weight, measured by viscosimetry, of at least 1.times.10.sup.6, in particular 2.5.times.10.sup.6 to approximately 1.times.10.sup.7. A process for determining such high molecular weights is described, for example, in CZ-Chemische Technik 4 (1974), 129 et seq.
UPE is distinguished by a range of physical characteristics which open it up to a variety of possible applications. Emphasis can be placed on its high wear resistance, its low coefficient of friction with respect to other materials, and its outstanding toughness. Furthermore, it is remarkably resistant to many chemicals. Because of its favorable mechanical, thermal, and chemical behavior, UPE has a wide variety of applications as a versatile material. Examples which can be quoted are the textile industry, mechanical engineering, the chemical industry, and mining.
The possible uses of this material are limited by the fact that its processing in ram extruders and the conventional single-screw or multi-screw extruders does not always lead to satisfactory results. In order to extrude UPE as gently as possible, i.e. without impairing its mechanical properties, ram extruders are widely used. Despite its many advantages, this processing method does not meet all requirements. In particular, the ram-stroke marks occurring on the molding are troublesome and not always acceptable.
The hollow bodies and profiles produced from UPE on screw extruders do not have these disadvantages. However, the polymer is greatly overheated, even at medium screw speeds. As a consequence of the high viscosity of the melt, which is only slightly reduced even with increased temperature, a very large proportion of the mechanical energy supplied to the screw is transformed by friction into heat. The heating of the material which this causes can lead to thermal damage to the plastic as a result of degradation or decomposition, i.e. by cleavage of the molecular chains, and thus to a reduction in the average molecular weight. Whereas the throughput, i.e. the amount of extrudate transported per unit time, increases approximately proportionally to the screw speed, the temperature increases at a rate which is disproportionately higher. For this reason, UPE can only be extruded on screw extruders of conventional design at low screw speed. However, the process then becomes uneconomical and unsuitable for many applications.
EP-A-01 90 878 discloses the processing of ultra-high molecular weight polyethylene in a single-screw extruder. The process consists in extruding molten UPE through a die with a length/diameter ratio of at least 10. The extrudates are taken off with a stretch ratio of at least 1, preferably 8 to 30:1. This method is only suitable for producing stretched filaments with small diameters at very low output rates.